The present invention pertains to a cleaner for electrical contacts. More specifically, the present invention pertains to a propellant-rich aerosol cleaner for use on electrical circuit boards, electrical contacts and the like.
Electrical contact cleaners are used during the manufacture of electronic, telecommunications and other electrical equipment to clean the components prior to final assembly. These cleaners are also used during maintenance operations carried out on electrical equipment in order to provide for proper electrical conductivity where two conductive (e.g., metal) surfaces are to be joined to one another in electrical contact. These cleaners are used to remove grease and more particularly other contaminants such as light oils, oxidation products and the like.
Many such cleaners are provided in aerosol form. These aerosol cleaners include a solvent and a propellant. Typically, aerosol formulations are solvent-rich. That is, these cleaners include high concentrations of solvent relative to propellant. The solvent is that portion of the cleaner that dissolves or loosens the contaminants while the propellant is that portion of the cleaner that is used as a vehicle to communicate the solvent.
Early contact cleaners used a solvent of CFC-113 (Freon(copyright) TF, trichlorotrifluoroethane) as a solvent of choice. CFC-113 was an ideal candidate in that it had many advantageous characteristics, such as no flash point, no explosion limits, good solvency for hydrocarbon and fluorinated soils, and, it was believed, low toxicity. In addition, and importantly, it was found to be safe to use on plastics, such as those used for circuit board substrates. In addition, CFC-113 was found to evaporate quickly thus making it an ideal solvent. It was, however, found that CFCs, generally, and other halogenated substances could accelerate the destruction of stratospheric ozone.
As a result, numerous CFC-free contact cleaners were developed. One such cleaner was fluorodichloroethane (HCFC-141b) which became available in commercial quantities. While it was found that HCFC-141b had many of the advantageous properties of CFC cleaners, it was also found that HCFC-141b attacked certain plastics and also had an ozone depletion potential, albeit less than that of CFC-113. Other cleaners were subsequently developed including hydrofluoroethers (HFEs) and perfluorocarbons (PFCs) which, while having advantageous properties with respect to cleaning contamination from electrical circuit boards were found to be too expensive to be generally accepted in the marketplace.
With respect to these cleaners, there are a number of desirable physical and chemical properties and characteristics that these cleaners should exhibit. These include low flammability, high evaporation rate, excellent cleaning performance, good wetting, high dielectric breakdown and a reasonable level of plastic compatibility. In addition, these cleaners must be compatible with known aerosol delivery systems and must continue to provide these physical and chemical characteristics or properties and delivery system characteristics in a relatively low cost, low toxicity product that is commercially viable.
With respect to the propellants used in such cleaners, typically these cleaners use a pressurized gas, such as carbon dioxide (CO2) as the propellant. CO2 is non-flammable, relatively low in cost and provides adequate flow rates and atomization of the solvent. In a typical application, the carbon dioxide is used in a weight percent of about 3 percent to about 5 percent.
Another known propellant is a non-flammable HFC liquefied gas, such as tetrafluoroethane (HFC-134a). Generally, HFC-134a is used in solvent-rich cleaners and is found in weight percents of about 20 percent to about 30 percent by weight. HFC-134a is, however, a more expensive propellant than is CO2.
One known propellant-rich cleaner includes a high-alcohol content solvent formulation. As such, it exhibits an unacceptably high residual flame characteristic that could render it unsafe for use in certain applications in which low residual flame properties are desirable.
Accordingly, there exists a need for an electrical contact cleaner that utilizes a solvent having good cleaning properties, as well as low toxicity, low or no ozone depletion potential and a degree of plastic compatibility. Desirably, such a cleaner uses a propellant that, in conjunction with the solvent, maintains low to no flammability and high evaporation rates and is acceptably priced for the marketplace. Most desirably, such a cleaner is propellant-rich and performs as well if not better than CFC based products.
A propellant-rich aerosol cleaner for use with electrical circuit boards and electrical connector components is formed from a solvent having a low or no ozone depletion potential and a propellant. The solvent is present in a concentration of at most about 50 percent by weight of the cleaner and the propellant is present in a concentration of at least about 50 percent by weight of the cleaner. Preferably, the solvent is present in a concentration of about 20 percent to about 40 percent by weight of the cleaner and the propellant is present in a concentration of about 60 percent to about 80 percent by weight of the cleaner. The cleaner exhibits cleaning characteristics of about 0.25 milligrams of soil per gram of solvent delivered and exhibits no residual flammability.
In a preferred formulation, the solvent is n-propyl bromide. The solvent can further include n-propanol. Preferably, the propellant is present in a concentration of about 60 percent by weight of the cleaner and the solvent is present in a concentration of about 40 percent by weight of the cleaner. In a current formulation, the n-propyl bromide is present in a concentration of about 37 percent by weight of the cleaner and the n-propanol is present in a concentration of about 1 percent to about 5 percent by weight of the cleaner.
In this formulation, the aerosol cleaner uses an HFC liquefied gas as a propellant. Preferably, the HFC liquefied gas is tetrafluoroethane and is present in a concentration of about 60 percent by weight of the cleaner.
In an alternate formulation, the aerosol cleaner includes trichloroethylene in a concentration of about 20 percent by weight of the cleaner as the solvent. In this formulation, the solvent can further include isopropyl alcohol in a concentration of about 2 percent to about 4 percent by weight of the cleaner. A preferred propellant is tetrafluoroethane in a concentration of about 77 percent by weight of the cleaner for this alternate formulation.
In still another alternate formulation, the aerosol cleaner uses either trichloroethylene or trans 1,2-dichloroethylene at a total concentration of about 7 percent to about 9 percent by weight in combination with a hydrofluoroether at about 21 percent by weight of the cleaner as the solvent. An acceptable hydrofluoroether is commercially available from 3M Company of St. Paul, Minn. under the tradename HFE 7100. In these formulations the solvent can further include isopropyl alcohol in a concentration of about 2 percent to about 4 percent by weight of the cleaner. A preferred propellant is tetrafluoroethane in a concentration of about 70 percent by weight of the cleaner for this alternate formulation.
Other features and advantages of the present invention will be apparent from the following detailed description, in conjunction with the appended claims.